1. Field of the Invention
The present invention relates generally to an X-ray source with a focused electron beam. More particularly, the present invention relates to a miniature magnetic electron lens assembly for reducing the cross-section of an electron beam at its intersection with the anode target of a miniature, mobile X-ray tube, thereby reducing the size of its x-ray emitting region and increasing the local intensity of its X-ray output.
2. Related Art
In an X-ray tube, electrons emitted from a cathode are accelerated toward an anode by an electric field produced by a bias voltage maintained between the two electrodes. The intervening space must be evacuated to avoid electron energy loss and scattering through collisions with gas atoms or ions and to prevent ionization of containment gas and the subsequent acceleration of positive ions to the cathode, where they can damage the electron emission source and limit tube life. Characteristic and Bremsstrahlung X-rays are generated by electron impact upon the anode target material. Every material is relatively transparent to its own characteristic X-radiation, so if the target is sufficiently thin, there may be strong X-ray emission through the target material, exiting the surface of the target that is opposite the electron impact site. A device in this configuration is termed a transmission type, or “end window”, X-ray tube. By comparison, a bulk anode tube, or “side-window” tube, has a thick, non-transparent target in the vacuum space, and its X-ray emission passes from the tube via an X-ray transparent window placed in the side of the vacuum chamber, as if reflected from the surface of electron incidence. Each anode type has its advantages and disadvantages, depending upon the intended application.
Typical high-power X-ray tubes are somewhat bulky and fragile. Such X-ray tubes must be energized by large, high-voltage power supplies that limit the mobility of the devices. Generally, specimens must be collected and brought to the stationary X-ray source for analysis. This is inconvenient for many X-ray applications. Certain “field applications”, for which it is advantageous to take the instrument to the sample, rather than the sample to the instrument, include X-ray fluorescence (XRF) of soil, water, metals, ores, well bores, etc., as well as X-ray diffraction and material thickness measurements.
For low-power X-ray applications, such as XRF, one popular approach to device portability is the use of 109Cd as the x-ray source. This radioactive isotope of cadmium emits X-rays as a result of nuclear decay. There are many instruments incorporating radioactive cadmium currently in use, and methods have been developed to make XRF analysis with the energy emitted by the isotope sensitive and reliable. Unfortunately, the intensity of emission from 109Cd decays exponentially, with a half-life of about 1.2 years. This necessitates frequent recalibration and eventual disposal of the isotope source. In addition, the radioactivity of a cadmium source suitable for XRF is approximately 1-2 Curies, so a license is required for transportation and possession of the isotope at the quantity and activity level required.
Miniature, non-isotope X-ray tubes have been demonstrated for medical purposes. For example, see U.S. Pat. Nos. 5,729,583 and 6,134,300. The geometry of the referenced devices, however, is not ideal for miniature, portable XRF analysis. These medical X-ray tubes are designed to send radiation into at least π steradians and to irradiate a relatively large specimen area for therapeutic reasons, rather than concentrating it into a beam or spot that is easily accessed by a detector. Thus, medical X-ray tubes are inadequate for most in-situ XRF analysis because of the divergence of their X-ray output. Another type of medical tube is a combination device in which the X-rays are used for diagnostic purposes, with the source placed inside the patient's body. Emitted X-rays pass through tissue to film that is external to the body, revealing the position of tumors or anatomic maladies. For example, see U.S. Pat. Nos. 5,010,562 and 5,117,829. With respect to the '562 patent, it is important to note that the foil is not a transmission type anode, but an electron window. With respect to the '829 patent, an interesting nozzle is shown, but the rest of the apparatus is large and inadequate for mobile fieldwork.
Another type of X-ray tube includes a rod anode used for insertion into pipes and boilers for X-ray inspection. The evacuated anode is hollow from the point at which the electron beam enters to the target surface at the opposite end. The whole rod structure is electrically biased at the anode potential. A window in the side of the rod allows X-rays to be emitted from inside the device. To focus the electron beam on the target at the end of the rod furthest from the cathode, an external magnetic coil is positioned coaxial with the rod, along its entire length. The electromagnet is heavy and requires considerable power from a large battery, if it is to be mobile. Additionally, the long anode of this configuration offers no benefit to typical analytical applications.
To obtain a concentrated source of X-rays at the anode of an X-ray tube, electron optics including lenses and apertures are usually employed. These optical elements are designed to focus the electron beam to a small diameter on the target, reducing the apparent size of the X-ray source. One example of such an optical element is a Wehnelt aperture, often used near the cathode of an electron microscope. A drawback of the Wehnelt aperture is that it significantly limits electron flux exiting the cathode. For XRF it is more important to limit the diameter of the electron beam where it strikes the anode, rather than at the cathode, since the anode is the site for the generation of X-rays directed at a (preferably) small portion of the analyte. The requirement of a small beam cross-section at the anode typically calls for other electrodes to act as beam-focusing elements. One example of such an element is a hollow, cylindrical focusing electrode spanning approximately half the distance from the cathode to the anode. An arrangement of this kind can be regarded as an electron lens. The field-shaping electrode, in effect, reduces the distance between anode and cathode, however; and it can increase the risk of electrical breakdown inside the X-ray tube.
An important feature of any device used to excite X-ray fluorescence for elemental analysis is that the point where the X-rays are generated should be as close as possible to the sample being irradiated. This is necessary, because the intensity of the X-rays decreases in proportion to the reciprocal of the square of the distance from the target. It is a further advantage to XRF analysis if the X-ray flux is focused to a small spot on the sample, for reasons of spatial resolution. A small X-ray source allows analysis of discrete, small portions of a complex sample.
In XRF, the X-ray beam is used to excite elements in the sample. The elements, in turn, fluoresce characteristic radiation in a Lambertian spatial distribution; so XRF sensitivity is maximized, if instrument geometry permits an angle of about 45° between the beam illuminating the analyte and the fluoresced X rays going into the detector. For generic X-ray tubes, the large apparent size of the x-ray source requires that the detector must be placed to one side, with an angle that is 90° or more instead of the desired 45° with respect to the incident radiation.
An object of the Treseder patent (U.S. Pat. No. 6,075,839) is to make the target accessible to the sample, but the exit window end of this invention is necessarily quite broad (greater than 20 mm). In addition, the anode is greatly recessed from the window, because the tube's electron gun is placed at the side of the anode instead of generally behind it. Moreover, it is impossible to modify the Treseder design to remedy this geometric disadvantage, because the target must be well separated from the X-ray window to make room for the curvature of the electron beam. The result is a large distance between the target and the sample, as shown in FIG. 3 of that patent.
Another requirement for sensitive XRF is that the sample be irradiated with X-rays of the correct wavelength or wavelengths for the material under inspection. Higher bias voltage not only increases X-ray flux, but it changes the energy distribution, or spectral content, of the output. Preferably, the anode-to-cathode bias voltage should be selectable by the operator and should be controlled independently of the anode to cathode current setting. In general, the higher the X-ray flux (and corresponding beam current), the more sensitive and accurate will be the measurements performed with the device, whether they are for XRF, material thickness measurement or X-ray diffraction. Only once the detector becomes saturated does additional X-ray flux offer no advantage. The current of the electron beam should, therefore, be adjusted independently of the acceleration voltage to provide adequate, but not excessive, X-ray intensity.
For generic X-ray tubes, substantial cooling is required, because the generation of X-rays by electron impact is a very energy-inefficient process. Less than one percent of the kinetic energy of the electron beam is actually converted to X-rays. The rest of the energy is converted to heat in the target. Heat is also generated by a thermionic electron source (i.e. a filament), if present. The heat generated in an X-ray tube should not, however, be permitted to substantially elevate the temperature of the device, because the lifetime of several tube components decreases with increasing temperature. Thermal shock, accompanying rapid changes in temperature, is also a particular concern. For thermal considerations, most X-ray tubes need to be cooled with a flowing liquid or forced air while in operation. Cooling effectiveness is limited primarily by the thermal conductivity of the bulk of the tube (e.g. the anode, in particular). Miniaturization mitigates this problem to some extent, but cooling is still required for the inventions of U.S. Pat. No. 6,075,839 (cooling by oil, SF6, or forced air) and for U.S. Pat. No. 6,044,130, which has exterior protrusions to aid in cooling by forced air. To achieve sufficient X-ray flux, conventional X-ray tubes must be so large that they require active cooling. A sufficiently powerful tube, cooled by heat exchange with ambient air alone, is not common for any application.
Furthermore, the X-ray output of many X-ray tubes can, in general, suffer from variation in the size, shape and location of the electron beam cross section, or “spot”, at the anode target. The electron beam can be relatively unfocused, misshapen, poorly positioned (i.e. off-axis) or subject to movement relative to the target, resulting in poorly concentrated, low level, or unstable output. This is a clear disadvantage to an analytical application, such as XRF, requiring stable, moderate levels of X-ray emission.